1. Field of the Invention
This invention relates to an optical transmitter in which a laser is used.
2. Description of the Related Art
Conventionally, an optical transmitter using a laser employs a compensator which compensates for variations of threshold current and differential quantum efficiency due to temperature variation or deterioration with the passage of time in an output of the laser.
For example, an automatic power control (hereinafter referred to simply as APC) is disclosed in Japanese Patent Laid-Open No. 90673/1993 that a control loop which receives and converts back light of a laser into a current by means of a photodiode, converts the current into a voltage, determines an average value of the voltage and controls a bias current with the average value and another control loop which detects a peak value of the voltage converted from the output current of the photo-diode and controls a modulation current for driving the laser with the peak value.
FIG. 1 is a block diagram showing an example of a conventional optical transmitter circuit having a function of compensating for variations of threshold current and differential quantum efficiency caused by temperature variation and deterioration with the passage of time of a laser.
Referring to FIG. 1, input signal 1 from an input terminal is applied to driver 43, and laser 4 is driven by driver 43 and bias circuit 42. Part of an output (back light) of laser 4 is monitored and converted into a current by photodiode 5, and the current is converted into a voltage by current/voltage converter 45.
An average value of the output voltage of current/voltage converter 45 obtained by first average value detector 47 and an average value of input signal 1 obtained by second average value detector 48 are compared with each other and the difference between them is amplified by amplifier 49, and bias circuit 42 is controlled with the output of amplifier 49.
Meanwhile, a peak value of the output from current/voltage converter 45 is detected by peak value detector 46, and driver 43 drives laser 4 with the peak value so that laser 4 outputs light.
With the optical transmitter described above, however, in order to compensate for a characteristic variation by temperature variation or deterioration with the passage of time of the laser output, it is required to convert the current for monitoring back light of the laser into a voltage and detect the peak value of the voltage, and this restricts the modulation rate of the optical transmitter.
The reason is that, where the modulation rate is higher than a certain level, the peak value cannot be detected by the peak value detector due to a limitation in frequency characteristic of the photodiode which monitors the back light of the laser.